Addition of phosphate group in the presence of light when undergoing a light reaction is called Photophosphorylation. It is a part of photosynthesis where ADP is converted to ATP to ultimately provide energy to a reaction. Cyclic and non cyclic Photophosphorylation are the two main classification of this reaction. In cyclic Photophosphorylation photosystem 1 (PS1) is used, where the electron are passed back into the photosystem producing ATP only. Non-cyclic Photophosphorylation uses both photosystem (PS1, PS2) the electrons are passed via electron carriers to photosystem. Both ATP and reduced NAD are produced.
Cyclic-Photophosphorylation
A sole photosystem is concerned in bacterial photosynthesis. When an electron is energized by absorption of light, it is ejected from the photosystem reaction center. The electron then passes through ferredoxin, and then down through the cytochrome b6F complex, Plastocyanin (pC), and finally back to the reaction center. Electron transport gives energy when a proton is generated this energy is utilized to produce ATP. As the excited electron returns to the reaction center this mechanism is termed as cyclic phosphorylation. In this mechanism NAD for biosynthesis is not generated.
Noncyclic Photophosphorylation
Two photosystem are used by plant and cynobacterium to produce both energy and reducing power. First a photon of light ejects high energy electron from photosystem 2. The electron lost from the photosystem does not return to photosystem but is replaced by the electron generated by splitting of water and production of oxygen. That is why it is called Non-cyclic Photophosphorylation. Then from excited reaction center the electron moves towards plastoquinone, to the b6F complex to Plastocyanin and finally to reaction center of photosystem 1. An electron motive force is generated by this electron transport system that is used to produce ATP. When photosystem 1 absorbs a photon of light it ejects a high energy electron, the energy from this process is used to generate the reducing power source called NADPH. The ejected electron is replaced by an electron of photosystem 2.
Key Differences
- In cyclic phosphorylation lone photosystem 1 is concerned while both photosystem 1 and 2 are used in Non-cyclic Photophosphorylation.
- The active reaction center of cyclic Photophosphorylation is P700 while the active reaction center of Non-cyclic Photophosphorylation is 680.
- In cyclic Photophosphorylation the electron travels in a cyclic manner while in non-cyclic Photophosphorylation it does not complete the circuit.
- In cyclic Photophosphorylation electron travels back to PS1 while in non cyclic Photophosphorylation NADP takes away the electron.
- ATP is produced in cyclic Photophosphorylation while NADPH and ATP both are produced in non cyclic Photophosphorylation.
- Cyclic Photophosphorylation takes place in isolated chloroplast and bacterial photosynthesis while in Noncyclic green plants are involved.
- Photolysis or water splitting is absent in cyclic Photophosphorylation while it is present in non cyclic Photophosphorylation.
- In cyclic Photophosphorylation oxygen is not evolved while in non cyclic Photophosphorylation oxygen is produced.
- Cyclic Photophosphorylation is predominantly seen in bacteria while non-cyclic Photophosphorylation is predominantly seen in green plants.
- There is no effect of DCMU on cyclic Photophosphorylation while DCMU inhibits the non cyclic Photophosphorylation.